Total disc replacement device

ABSTRACT

The total disc replacement device ( 1 ) comprising a central axis ( 2 ), a first and a second apposition plate ( 3; 5 ), a first and a second joint component ( 4; 6 ) being mutually arranged in a ball joint like manner and being located between said first and second apposition plates ( 3; 5 ) and intermediate means ( 7 ) being disposed at least between the first apposition plate ( 3 ) and the first joint component ( 4 ) in such manner that, the first joint component ( 4 ) is not freely moveable transversely to the central axis ( 2 ) with respect to the first apposition plate ( 3 ) under load-free conditions, but a limited movement of the first joint component ( 4 ) relative to the first apposition plate ( 3 ) transversal to the central axis ( 2 ) is allowed under load.

The invention relates to a total disc replacement device comprising afirst and a second apposition plate, a first and a second jointcomponent and intermediate means disposed between the first appositionplate and the first joint component.

Today implants or prostheses are inserted into the intervertebral spaceof two adjoining vertebral bodies after removal of a naturalintervertebral disc or a nucleus pulposus of an intervertebral disc.

Usually hard material inlays for articulating components of total discreplacement implants are manufactured from super alloys or ceramicmaterials. These inlays are usually pressed or shrunk into titanium orsuper alloy bone contact plates, whereby the interface is subject tosevere compression stresses.

Total disc replacement devices comprise components for the preservationof segmental motion. Some total disc replacement designs include one ormore joints having spherical articulation, e.g., ball-and-socket joint.Materials used to construct the articulating elements can comprisecombinations such as metal-on-polymer, metal-on-metal orceramic-on-ceramic bearing as well as the combination of thesematerials. Generally, the material combinations aim for minimal weardebris volume and biocompatible wear particles.

An intervertebral implant is known from DE 2003 13 183 AESCULAP, whereinthe articulating elements are arranged in an axially rotatable manner inthe bone contact plates. The rotation of the articulating elements inthe bone contact plates shows the disadvantage of abrasion and a higherclearance.

In embodiments where any rotation should be avoided, inlays aremanufactured from hard materials (e.g., metal, metal alloys, and/orceramic) that have various disadvantages when assembled or otherwiseplaced into the bone contact plates. Usually the inlay has overlappingdimensions or dimensions that allow the inly to be assembled into thecomponent with a press-fit. However, press-fitting results in highcompression forces that can stress the inlay and other components.

A metal inlay may be designed so that it deforms during a press-fit,resulting in a reduction of the clearance of the articulation. Highcompressive stress to ceramic elements is typically unproblematic iftheir design is structurally strong. However, total disc replacementdevices are relatively small which usually prevents the use of a thickwall design. Since the ceramic material is brittle, and press- orshrink-fitting a ceramic portion of a total disc replacement device candamage the ceramic portion and results in small fragments with sharpedges that could damage. The major blood vessels located near theintervertebral discs. Therefore, stress shielding, particularly to theceramic material, should be considered in the design.

On this point, the invention intends to provide remedial measures. Theinvention is based on the objective of providing a total discreplacement device, which allows a clearance-free connection between theapposition plate and the joint component corresponding thereto and whichis provided with a stress shielding capacity in order to reduce thecompression stress in the concerned interface.

In some embodiments, this invention provides a total disc replacementdevice that has a central axis and comprises a first and a secondapposition plate, a first and a second joint component, and anintermediate means that is disposed at least between the firstapposition plate and the first joint component. The first and secondjoint component can located between the first and second appositionplates. The first and second joint components can be mutually connectedor arranged in a ball-joint like manner. The intermediate means can bedisposed at least between the first apposition plate and the first jointcomponent in such manner that the first joint component is not freelymoveable transversely to the central axis with respect to the firstapposition plate under load-free conditions, but a limited movement ofthe first joint component relative to the first apposition platetransversal to the central axis is allowed under load.

One advantage of the present invention is that it reduces or eliminatescompressive stress on the various components of the total discreplacement device. For example, the intermediate means that can bedisposed between an apposition plate and a joint component can reduce oreliminate the compressive force needed to assemble the device. Also, theintermediate means can reduce or eliminate the stresses born by thecomponents when implanted into a patient.

Additional advantageous embodiments of the invention are characterizedbelow.

In one embodiment of the invention at least one of the intermediatemeans is connected to the corresponding apposition plate or to thecorresponding joint component with a press-fit. The advantages of suchembodiment is to be seen in the facts that stresses due to the highcompression forces between the adjacent components are reduced,clearance between the components is avoided and the respective membersare firmly kept together. Due to the choice of the soft, elastic orthermoplastic properties of the intermediate means stress can be reducedsince the softer component adapts its shape in order to establish astress-balance.

In a further embodiment the intermediate means are elastically orplastically deformable so that a limited movement of the jointcomponents relative to the corresponding apposition plates is allowedunder load.

In another embodiment the press-fit of the intermediate means within theadjacent apposition plate or within the adjacent joint component isobtainable by an elastic deformation of the intermediate means. Due tosuch embodiments stress shielding is obtainable e.g. with insertion of apolymeric ring.

In a further embodiment the press-fit of the intermediate means withinthe adjacent apposition plate or within the adjacent joint component isobtainable by a plastic deformation of the intermediate means. Due tosuch embodiments stress shielding is obtainable e.g. with deformablemeans being integral with the adjacent apposition plate or the adjacentjoint component.

In another embodiment the apposition plates are connected to thecorresponding joint component by a plug-socket connection, such that atleast a part of the apposition plate and at least a part of thecorresponding joint component overlap one another coaxially to thecentral axis and form an attachment zone, whereby the intermediate meansis disposed between the apposition plates and the corresponding jointcomponent only within the attachment zone preferably within the wholeattachment zone.

In a further embodiment the intermediate means has a height beingsmaller than a height of the attachment zone. The advantage of suchembodiment can be seen in the fact that due to small height of theintermediate means a major deformation of said intermediate means isallowing.

In another embodiment the Young's modulus of the material of theintermediate means is lower than that of the adjacent apposition plateand/or lower than that of the adjacent joint component. Due to suchembodiment the total disc replacement device may comprise components,particularly apposition plates and/or joint components may be made of amaterial having a high Young's modulus, e.g. use of titan or titanalloys being biocompatible or use of ceramics for the joint components.

In another embodiment the intermediate means consist of the materialbeing selectable of the group of PEEK, PEKK or others members of thePEAK. The advantage of such embodiment can be seen in the fact thatthermoplastic semi-crystalline polymers comprise elastic properties witha low grad of creep (alignment of macro-molecules under load). Duringand after the insertion of the stiff inlay, the intermediate means isable to adapt its shape and distribute the interface stress. Since thisprocess is performed slowly, the risk of rapid load changes is reduced.PEEK, PEKK and other members of the PEAK family, even re-enforced withcarbon fibres, PTFE, UHMWPE, all materials must be biocompatible.

In a further embodiment the apposition plates consist of the materialbeing selectable of the group of titanium and its alloys, CoCrMo, orstainless steel and the joint components consist of the material beingselectable of the group of ceramics, preferably aluminium oxide,aluminium titanate, silicate ceramics and zirconium oxide, or of thegroup of CoCrMo or CoCrMoC alloys.

In an additional embodiment the apposition plates and the adjacent jointcomponents consist of the same material. If the design of the appositionplates would be sophisticated (“filigrane”), the use of a material withhigher Young's modulus is indicated, i.e. CoCrMo alloy.

In a further embodiment said intermediate means has an essentiallycylindrical, preferably circular-cylindrical shape. The advantages ofsuch embodiments can be seen in the facts of uncomplicatedmanufacturing; homogeneous stress distribution; easier assembly of thesnap fit. Furthermore, if the inlay is inserted in-situ (in theintervertebral space), the surgeon is independent from the axialposition of the components.

In another embodiment the intermediate means consist of a memorymaterial. After insertion of the intermediate means, they can be heatedlocally and find its final shape and position.

In a further embodiment said intermediate means and the adjacentapposition plate or the adjacent joint component are connectable bymeans of a snap-fit, whereby said apposition plates or joint componentsare provided with lip and said intermediate means are provided with acorresponding undercuts such forming a snap-fit. In another embodimentsaid apposition plates or joint components are provided with undercutsand said intermediate means are provided with a corresponding lip suchforming a snap-fit. The advantage of the additionally form-lockingconnection of the intermediate means and the corresponding componentscan be seen in the fact of the prevention of the spontaneous dismantlingand easy manufacturing of all components allowed thereby.

In a further embodiment the intermediate means has a spring-shape beingcorrugated transversely to the central axis. Such corrugated shapeallows the use of a metallic material instead of a thermoplasticpolymer.

In another embodiment the intermediate means and the correspondingapposition plate or the corresponding join component consist of onepiece. The advantage of such embodiments can be seen in the fact of thetension distribution and the reduction of the high loading pressure.

In a further embodiment the intermediate means are designed as pluralityof projections, whereby such projections are T-shaped or L-shaped.

The invention and additional configurations of the invention areexplained in even more detail with reference to the partially schematicillustration of several embodiments.

FIG. 1 illustrates an exploded view of an embodiment of the total discreplacement device according to the invention;

FIG. 2 illustrates a cross-section through the embodiment of FIG. 1;

FIG. 3 illustrates an exploded view of another embodiment of the totaldisc replacement device according to the invention;

FIG. 4 illustrates a cross-section through the embodiment of FIG. 3;

FIG. 5 illustrates a top view on the first apposition member of thedevice of FIG. 3; and

FIG. 6 illustrates an exploded view of a further embodiment of the totaldisc replacement device according to the invention;

FIG. 7 illustrates a top view on the first apposition member of thedevice of FIG. 6; and

FIG. 8 illustrates a top view on a further embodiment of the firstapposition member of the total disc replacement device;

FIG. 9 illustrates a cross-section through the first apposition plate ofthe total disc replacement device;

The embodiment of the total disc replacement device 1 is shown inFIG. 1. Device 1 comprises a central axis 2, a first and a secondapposition plate 3;5, a first and a second joint component 4;6, a firstand second intermediate means 7;9.

When device 1 is implanted in a patient, central axis 2 lies essentiallyparallel to a longitudinal axis of the patient's vertebra. First andsecond apposition plate 3;5 are disposed transversely to central axis 2.First and second joint components 4;6 are located between said first andsecond apposition plates 3;5. Together, first and second jointcomponents 4;6 form a ball-joint and are mutually connectable in aball-joint like manner. First intermediate means 7 is disposed betweenfirst apposition plate 3 and first joint component 4. Secondintermediate means 9 is disposed between second apposition plate 5 andsecond joint component 6.

First and second apposition plate 3;5 can be made of titanium or atitanium alloy. First and second joint component 4;6 can be made of aceramic. The first and second intermediate means 7;9 can be made of anelastic materials, such as, for example a rubber or other type ofpolymer (e.g., PEEK).

Said first apposition plate 3 comprises a first contact surface 31 and afirst intermediate surface 32. Accordingly, said second apposition plate5 comprises a second contact surface 51 and a second intermediatesurface 52. Both contact surfaces 31;51 as well as both intermediatesurfaces 32;52 are arranged transversely to said central axis 2. Saidfirst and second apposition plates are arranged relative to each otherin a manner that the first and second intermediate surface 32;52 areturned to each other and the first and second contact surfaces 31;51,which is apt to abut the adjacent vertebral bodies, are turned away fromeach other. Additionally, said first and second apposition plates 3;5each are provided with a three-dimensional structuring having a form ofe.g., spikes and allowing an anchorage of said first and secondapposition plates 3;5 in the adjacent vertebral bodies. Said first jointcomponent 4 has a spherical convex sliding surface 41 being arrangedconcentrically to said central axis 2 and oppositely a first end portion42 connected to said first apposition plate 3. Said second jointcomponent 6 is provided with a spherical concave sliding surface 61(FIG. 2) being slideably arranged, on said convex sliding surface 41 andopposite to said concave sliding surface 61 a second end portion 62connected to said second apposition plate 5. In this manner, the mutualarrangement of concave sliding surface 61 and convex sliding surface 41provide device 1 with ball-joint like movement.

Said first joint component 4 comprises circular cylindrical projection43 being coaxial to the central axis 2 and being disposed at said firstend portion 42 of said first joint component 4, whereby said firstapposition plate 3 comprises the first recess 33 penetrating from saidfirst intermediate surface 32 into said first apposition plate 3. Inthis manner, said circular cylindrical projection 43 may be insertedinto the first recess 33 such said circular cylindrical projection 43and said first recess 33 forming a first attachment zone 10 (FIG. 2).Furthermore, said second joint component 6 has a circular cylindricalsecond end portion 62 that may be inserted into the second recess 53penetrating from said second intermediate surface 52 into said secondapposition plate 5 such said second end portion 62 and said secondrecess 53 forming a second attachment zone 100. In this manner, each ofsaid first and second apposition plates 3;5 is connected to thecorresponding joint component 4;6 by a plug-socket connection, wherebythe connection between the apposition plates 3;5 and the correspondingjoint components (4;6) is clearance-free (i.e. no space between theapposition plates 3;5 and the corresponding joint components (4;6) isleft to avoid free motion between them).

Said first intermediate means 7 is configured as a ring radiallydisposed between said circular cylindrical projection 43 and said firstrecess 33, whereby said circular cylindrical projection 43 is pressedinto the central bore 71 of said first intermediate means 7. At thebottom surface 74 of said first intermediate means 7 the peripheralsurface 72 of said first intermediate means 7 comprises a circular lip73 forming a snap-fit with a circular undercut 11 at the bottom of saidfirst recess 33. Adjacent to said first intermediate surface 32 of saidfirst apposition plate 3 said first recess 33 has a diametricallyenlarged section apt to press-fittingly receive a first flange 75arranged at the top surface 76 of said first intermediate means 7.

Analogously, said second intermediate means 9 is configured as a ringradially disposed between said circular second end portion 62 and saidsecond recess 63, whereby said circular second end portion 62 is pressedinto the central bore 91 of said second intermediate means 9. At the topsurface 94 of said second intermediate means 9 the peripheral surface 92of said second intermediate means 9 comprises a circular lip 93 forminga snap-fit with a circular undercut 111 at the bottom of said secondrecess 53. Adjacent to said second intermediate surface 52 of saidsecond apposition plate 5 said second recess 53 has an diametricallyenlarged section apt to press-fittingly receive a second flange 95arranged at the bottom surface 96 of said second intermediate means 9.

FIGS. 3-5 show a further embodiment of the total disc replacement device1, whereby the intermediate means 7;9 are designed as a spring-likeelement. In contrast to the embodiment according to FIGS. 1 and 2comprising a cylindrically shaped intermediate means 7;9 the embodimentaccording to FIG. 3 does not require a circular lip forming a positivefit, but allows a frictional fit or connection between the intermediatemeans 7;9 and the corresponding apposition plates 3;5. FIG. 5 shows theintermediate means 7 being frictionally locked in the first recess 33 ofthe first apposition plate 3. After the insertion of the cylindricalprojections 43 of the first joint component 4 into the first recess 33of the first apposition plate 3 respectively the second end portion 62of the second joint component 6 into the second recess 53 of the secondend apposition plate 5 the intermediate means 7;9 acting as a spring areplastically or elastically deformed and are clamped over the first andsecond attachment zones 10;100 with a press fit (FIG. 4).

FIG. 6 shows a further embodiment of the total disc replacement device1, whereby the intermediate means and the corresponding appositionplates consist of one piece and the intermediate means are designed as aplurality of projections 50 formed in a T-shape configuration and beingdisposed on the periphery of the first and second recess 33;53. FIG. 7shows a top view on the first apposition member 3 of the total discreplacement device according to FIG. 6 with the first recess 33 beingprovided with T-shaped projections 50 on the periphery of said recess33.

FIG. 8 shows a top view of a further embodiment of the first appositionmember of the total disc replacement device with the intermediate meansand the first apposition plate 3 consisting of one piece, whereby theintermediate means are preformed as a plurality of L-shaped projections50.

FIG. 9 shows a cross-section through the first apposition plate 3 of afurther embodiment of the total disc replacement device 1, whereby theheight H of the intermediate means 7 is smaller then the height L of theattachment zone 10.

1. A total disc replacement device (1) having a central axis (2) andcomprising: A) a first and a second apposition plate (3;5); B) a firstand a second joint component (4;6) being mutually arranged in aball-joint like manner and being located between said first and secondapposition plates (3;5); and C) intermediate means (7) being disposed atleast between the first apposition plate (3) and the first jointcomponent (4) in such manner that the first joint component (4) is notfreely moveable with respect to the first apposition plate (3) underload-free conditions, but a limited movement of the first jointcomponent (4) relative to the first apposition plate (3) transversal tothe central axis (2) is allowed under load.
 2. The device (1) accordingto claim 1, wherein the connection between the apposition plate and thejoint component is clearance-free.
 3. The device (1) according to claim1 or 2, further comprising second intermediate means (9) being disposedbetween the second apposition plate (5) and the second joint component(6).
 4. The device (1) according to one of the claims 1 to 3, whereinsaid intermediate means (7) are elastically deformable.
 5. The device(1) according to one of the claims 1 to 3, wherein said intermediatemeans (7) are plastically deformable.
 6. The device (1) according to oneof the claims 1 to 5, wherein at least one of said intermediate means(7;9) is connected to the corresponding apposition plate (3;5) with apress-fit.
 7. The device (1) according to one of the claims 1 to 6,wherein at least one of said intermediate means (7;9) is connected tothe corresponding joint component (4;6) with a press-fit.
 8. The device(1) according to one of the claims 1 to 7, wherein the press-fit of atleast one of the intermediate means (7;9) within the adjacent appositionplate (3;5) or within the adjacent joint component (4;6) is obtainableby an elastic deformation of the intermediate means (7;9).
 9. The device(1) according to one of the claims 1 to 8, wherein the press-fit of atleast one of the intermediate means (7;9) within the adjacent appositionplate (3;5) or within the adjacent joint component (4;6) is obtainableby a plastic deformation of the intermediate means (7;9).
 10. The device(1) according to one of the claims 1 to 9, wherein at least one theapposition plates (3;6) is connected to the corresponding jointcomponent (4;6) by a plug-socket connection, such that at least a partof the apposition plate (3;5) and at least a part of the correspondingjoint component (4;6) overlap one another coaxially to the central axis(2) and form an attachment zone (10;11).
 11. The device (1) according toclaim 10, wherein the intermediate means (7;9) is disposed between theapposition plate (3;5) and the corresponding joint component (4;6) onlywithin the attachment zone (10;11), preferably within the wholeattachment zone (10;11).
 12. The device (1) according to one of theclaims 1 to 11, wherein the Young's modulus of the material of theintermediate means (7;9) is lower than that of the adjacent appositionplate (3;5).
 13. The device (1) according to one of the claims 1 to 12,wherein the Young's modulus of the material of the intermediate means(7;9) is lower than that of the adjacent joint component (4;6).
 14. Thedevice (1) according to one of the claims 1 to 13, wherein theintermediate means (7;9) consist of the material being selectable of thegroup of PEEK, PEKK or others members of the PEAK.
 15. The device (1)according to one of the claims 1 to 14, wherein at least one of theapposition plates (3;5) consists of the material being selectable of thegroup of titanium and its alloys, CoCrMo, or stainless steel.
 16. Thedevice (1) according to one of the claims 1 to 15, wherein at least oneof the joint components (4;6) consists of the material being selectableof the group of ceramics, preferably aluminium oxide, aluminiumtitanate, silicate ceramics and zirconium oxide, or of the group ofCoCrMo or CoCrMoC alloys.
 17. The device (1) according to one of theclaims 1 to 16, wherein said apposition plates (3;5) and said adjacentjoint components (4;6) consist of the same material.
 18. The device (1)according to one of the claims 1 to 17, wherein said intermediate means(7;9) has an essentially cylindrical shape.
 19. The device (1) accordingto claim 18, wherein the intermediate means (7;9) has an essentiallycircular-cylindrical shape.
 20. The device (1) according to one ofclaims 1 to 3, 6 to 7 and 10 to 19, wherein at least one of theintermediate means (7;9) consists of a memory material.
 21. The device(1) according to one of the claims 1 to 20, wherein said intermediatemeans (7;9) and the adjacent apposition plate (3;5) or the adjacentjoint component (4;6) are connectable by means of a snap-fit.
 22. Thedevice (1) according to one of the claims 1 to 21, wherein saidapposition plates (3;5) or joint components (4;6) are provided with alip (73;93) and said intermediate means (7;9) are provided with acorresponding undercuts (11;111) such forming a snap-fit.
 23. The device(1) according to one of the claims 1 to 21, wherein said appositionplates (3;5) or joint components (4;6) are provided with undercuts(11;111) and said intermediate means (7;9) are provided with acorresponding lip (73;93) such forming a snap-fit.
 24. The device (1)according to one of claims 1 to 20, wherein the intermediate means (7;9)has a spring-shape being corrugated transversely to the central axis(2).
 25. The device (1) according to one of the claims 10 to 24, whereinthe attachment zone (10) has a height L of between 1,5 mm and 9 mmmeasured parallel to the central axis (2).
 26. The device (1) accordingto one of the claims 1 to 25, wherein said intermediate means (7; 9) hasa height H between 0,7 mm and 6,0 mm measured parallel to the centralaxis (2).
 27. The device (1) according to one of the claims 1 to 26,wherein said intermediate means (7; 9) has a minimum volume of 60 mm³.28. The device (1) according to one of the claims 1 to 27, wherein atleast one of said intermediate means (7;9) and the correspondingapposition plate (3;5) consist of one piece.
 29. The device (1)according to one of the claims 1 to 28, wherein at least one of saidintermediate means (7;9) and the corresponding join component (4;6)consist of one piece.
 30. The device (1) according to one of the claim28 or 29, wherein the intermediate means are designed as plurality ofprojections (50).
 31. The device (1) according to one of the claim 30,wherein the projections are T-shaped.
 32. The device (1) according toone of the claim 30, wherein the projections are L-shaped.